Question

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Python “is” operator behaves unexpectedly with integers

I’m learning Python, and am curious as to why:

x = 500
x is 500

returns False, but:

y = 100
y is 100

returns True?

Answer 1

Python reuses small integers. That is, all 1s (for example) are the same 1 object. The range is -5 to 255, if I remember correctly, though this is a CPython implementation detail that should not be relied upon. I am pretty sure Jython and IronPython, for example, handle this differently.

The reason this works out fine is that ints are immutable. That is, you can’t change a 4 to a 5 in-place. if a has a value of 4, a = 5 is actually pointing a to a different object, not changing the value a contains. Python doesn’t share any mutable types (such as lists) where unexpectedly having multiple references to the same object might cause problems.

You should use == for comparing most things. is is for checking to see whether two references point to the same object; it is roughly equivalent to id(x) == id(y).